Shunt capacitors are the most commonly used reactive power compensation apparatus in power systems. Connected between a bus and ground, each shunt capacitor injects capacitive reactive power into the system. One or more shunt capacitors can be used to boost the bus voltage, improve the system's power factor, and/or reduce power loss. Since a power system's reactive power needs often change, multiple switchable shunt capacitors can be connected to the same bus. Using switches, each of the capacitors can be connected or unconnected to the system based on system condition. This leads to a variable reactive power compensation apparatus. For example, if there are two switchable capacitors of 1 MVar and 1.5 MVar respectively, the total output of the apparatus can be 1 MVar (first capacitor on), 1.5 MVar (second capacitor on) and 2.5 MVar (both capacitors are on).
Although shunt capacitors are a simple and effective reactive power compensation apparatus, they suffer from the problem of resonance. Resonance occurs when a component's capacitive impedance is approximately equal to the system's inductive impedance at a particular frequency (e.g. 300 Hz). If the system has a current or voltage source at 300 Hz, resonance can occur. If resonance occurs, this can lead to a significant amplification of the 300 Hz voltage/current and eventual damage to the capacitors.
Power systems are designed to operate at one frequency, usually either at 50 Hz or 60 Hz. While voltage or current sources at other frequencies were very rare in the past, in recent years there has been an increase in the use of power electronic converters by utility customers. Such converters can inject currents (called harmonic currents) at frequencies that are integer multiple of the fundamental frequency. If a harmonic current has, for example, a frequency of 300 Hz (i.e. 5×60 Hz), a capacitor resonance may be excited or may occur. In the power industry, a resonance excited by or caused by harmonic currents or voltages is called a harmonic resonance. Shunt capacitors can thus be negatively affected by harmonic resonance.
Some solutions have been developed to mitigate the shunt capacitor related harmonic resonance. One method is to add a detuning inductor in series with the shunt capacitor. The detuning inductor changes the shunt capacitor into an inductive characteristic at the harmonic frequencies of concern, thereby mitigating the capacitor-system resonance. This method has been widely used in low and medium voltage systems due to its simple implementation and low cost. However, this method does not always work if the system exhibits capacitive impedance at certain frequencies. To address this issue, a damping element can be added to a shunt capacitor. The damping element's resistive component can damp the magnitude of resonance. U.S. Pat. Nos. 3,881,137, 3,555,291, 4,864,484, 5,805,032 disclose several topologies of damping elements to mitigate the capacitor-system resonance. Some utility companies have added damping elements to their high voltage shunt capacitors to mitigate harmonic resonance.
In addition to the above approach, R, L, or C components can be added in series with a shunt capacitor to turn it into a harmonic filter. For example, an inductor can be connected in series with a capacitor. The inductor is sized in such a way that the total impedance of the combined capacitor and inductor branch is zero at a harmonic frequency. Harmonic current at that frequency will flow to this low impedance branch. This branch thus becomes a standard single-tuned harmonic filter. Several approaches to converter a shunt capacitor into a harmonic filter have been reported in U.S. Pat. Nos. 3,038,134, 3,535,542, 4,406,991, 4,622,474, 4,939,486, 5,565,713, 5,668,418.
At present, the above resonance mitigation or harmonic filtering measures have been used for individual shunt capacitors. This means that, for multiple switchable capacitors, each of the capacitors requires a detuning, damping, or filtering element. The cost and space requirements for implementing these changes increase with the number of switchable capacitors.
There is therefore a need for systems, methods, and/or devices that mitigate if not overcome the shortcomings of the prior art.